A precipitant can form in less than 0.1 second.
A precipitate we can see, can be crystals (a salt of the metal and acid or base), that form from ions, Cations and Anions which grow to a growth of an ionic crystal large enough for us to see, the ions are so small we cannot begin to see them until they form large clusters of these ionic bonds to form a crystal of ions large enough for us to see, with many bonds of metal Cations and nonmetal Anions bound together to form a large enough crystal to become visible, and a heavy enough practical to sink with gravity.
With gold ions reduced to atoms (say from stannous chloride) the gold atoms are reduced from ions in solution to elemental gold atoms, individual atoms are to small for us to see, even clusters of thousands of these atoms can also be to small for us to see with the naked eye (we may be able to see them with some special microscope),.
It requires something like a cluster of 1000 atoms for the aggregate to be a colloidal particle, with a proximate diameter of 10-7cm .
Even with a diameter of 10-4cm, containing about a billion atoms the practical is still classified as colloidal and cannot be seen by our naked eye, without a microscope, although we can use a simple method to inspect the solution to see if it holds a colloid in suspension. By a beam of light from a light source like a flashlight, in a darkened room, the light beam viewed from the side as it passes through the solution in our beaker the colloids reflect the light to form a "glow". with gold different size of colloids can reflect different colors of light, color of the solution will appear different for different sizes of colloids (or the size of the clumps of invisible gold atoms) with stannous chloride the atoms clump together to the size that reflects a frequency of light we see as purple. it is not the atoms we see but a purple color of reflected light from the cluster of gold atoms.
Atoms or ions are too small for us to see, but if they join together into a large enough cluster we can see them, colloids are similar but they "develop a charge" before the ions or atoms clusters can grow into large enough of a cluster to become visible to us, stunted in their growth at a size too small to overcome gravity and settle in solution, these small clusters with the same charge repel each other keeping themselves in constant motion in solution shoving each other around, (like small magnets all with the same polarity), unable to come into contact with each other to grow to a larger crystal or clump of atoms, being repelled from each other, they cannot crash into each other to combine into a larger mass to become visible, or to gain in mass to become heavy enough to sink like a precipitant.
A precipitant is a mass of ions or atoms with grow to a large enough size to become visible and to sink.
The precipitant starts off as a colloidal size particles, these small particles crash into each other and combine to a larger crystal or clump of atoms, but does not form a colloid in solution and grows till it forms a visible precipitant or crystal or clumps of atoms we can see, becoming heavy enough to sink in solution. This happens almost instantly when two solution that form the precipitant are added together.
In colloids the ions crystal growth, or the cluster of atoms growth, is stunted before they become large enough to be visible, or heavy enough to overcome gravity in solution, the small clusters forming a polarizing charge as they grow to a certain size, and their growth is halted by the "charge", with a size too small for us to see, they begin to repel each other and will not grow to become visible, will not come together to form a larger particle, or become heavy enough to sink, pushing each around in solution almost indefinably, until something changes to break the colloid, or to remove the electrical charge that keeps them separated and from combining and growing into a precipitant, or dissolves them back into solution as ions.
